The present invention relates to heat-transfer technology and boiler-making and, more particularly, it relates to a multipass corrosion-proof air preheater.
The present invention can be used most advantageously in steam boilers, furnaces and like apparatuses which burn sulfur-bearing fuel and in whose air preheaters aggressive exit gases serve to heat up cool air.
In an air preheater, which is usually made multipass with respect to air supply, the first pass to which cool air is supplied is subject to intensive corrosion caused by the condensation of sulfuric acid vapors on relatively cool tube walls whose temperature depends on that of the exit gases and, even more, on the temperature of air flowing around the tubes. Inasmuch as the tubes of the first pass of the air preheater serve for the passage of the coolest gas and their outer surface is flown around with the coolest air, it is in the first pass that the tube wall temperature is rather low and, in part of the tubes, it is below the dew point of water vapors in flue gases containing sulfuric acid vapors. As a result, sulfuric acid condenses in various concentrations on the tube walls to cause intensive corrosion of metal and equally intensive crudding of tubes with ash deposits.
Due to corrosion of air preheater tubes, leading to local destruction of the latter, air from the tubefield gets into gases. Then, the capacity of the blow fan and smoke exhauster needs to be increased in order to obtain at the air preheater outlet the amount of air required for the boiler or furnace. This results in excessive consumption of energy. The energy consumption increases still further due to increased drag of the gas flow in tubes upon their clogging and crudding.
Later on, with the air preheater tubes being affected by corrosion, the blow fan and smoke exhauster capabilities are exhausted, as a result of which the boiler or furnace output needs to be decreased and, finally, the boiler or furnace is to be shut down for replacement of the damaged portion of the air preheater.
Well known in the art of boiler-making is a corrosion-resistant air preheater (cf., L. B. Krol, N. I. Rozengauz, Konvektivnyie elementy moschnykh kotelnykh agregatov - Convection Elements of Powerful Boiler Units, Energiya Publishers, Moscow, 1976, FIGS. 4-3, 4--4 pp. 132-133). Said prior art tubular air preheater is provided with a device for air preheat fashioned as a hot-air heater. Preheated air is supplied to the tubefield of the first pass. In such an air preheater, the tubes of the first pass, inside which relatively cool exit gases pass, are blown around on the outside with preheated air, as a result of which the wall tube temperature is higher than in the case of a conventional air preheater. Since the wall temperature is above the dew point of water vapors in exit gases, there is no condensation of moisture and, consequently, no corrosion and crudding of tubes are observed.
This air preheater is disadvantageous in that it effects adversely the efficiency of the boiler in which it is installed, due to increased temperature of exit gases leaving the air preheater caused by air preheat. Therefore, boiler makers only use such air preheater in cases when low air preheat is sufficient, i.e., when burning low-sulfur fuels featuring a low dew point of water vapors in exit gases. In burning high-sulfur fuels, high-temperature air preheat is required, however, this is accompanied by an inadmissibly high temperature of exit gases, and no increase of the heating surface is capable of appreciably cooling down the exit gases. In such cases, the above-considered air preheater appears inapplicable.
Also known in the art is a multipass corrosion-proof air preheater according to U.S. Pat. No. 4,243,096.
Said latter prior art air preheater has a plurality of passes series-connected, with respect to air supply, by means of connecting air lines. Each pass is formed by bundles of heat-transfer tubes designed for the passage of heating gases. The air preheater is provided with an air supply line connected with the pass that comes first in the direction of air flow. Mounted in the air supply line is a device for high-temperature preheat of pat of the air supplied to the tubefield of the bundles of heat-transfer tubes in the first pass. Upstream of the device for high-temperature air preheat, as viewed in the direction of air flow, at least one bypass air line is connected to the air supply line, said bypass air line serving to communicate the air supply line with the connecting air line via mixer installed in the latter.
In the latter prior art air preheater, only part of the air supplied to the first pass is subjected to high-temperature preheat in a special device such as air heater, while the remaining cool air enters the connecting air lines and a mixture of preheated and cool air at a temperature ensuring the wall tube temperature above the dew point of water vapors in exit gases is supplied to the subsequent passes of the air preheater. As a result, to each pass of the air preheater there is supplied highly preheated air ensuring the tube temperature above the dew point of water vapors in exit gases, so that no tube crudding and corrosion take place.
However, in spite of high-temperature preheat of air supplied to the first pass, the balance temperature of air supplied to the air preheater from the hot-air heater and bypass air line may be quite low and the mean temperature head of the air preheater stays high. Consequently, the exit gases can be cooled down appreciably, and the prior art air preheater requires no large increase of the heating surface for reducing the temperature of exit gases.
The latter prior art air preheater is deficient in that several bypass air lines need to be provided for increasing the mean temperature head by reducing the portion of highly preheated air supplied to the first pass, each one of said bypass air lines communicating the air supply line with each subsequent connecting air line via mixers mounted in the latter. This involves a complication of the general arrangement and structure of the air preheater, as well as lower reliability of its operation in view of the increased number of mixers in the connecting air lines. Moreover, such a structural arrangement of several bypass air lines results in a considerable portion of the air preheater heating surface being blown around with only part of the air, which affects the heat-transfer coefficient and mean temperature head of this surface of the air preheater. As a result, the overall efficiency of the air preheater is reduced.